Radio interferometer



M. BELLENGER RADIO INTERFEROMETER Oct; 3, 1967 3 Sheets-Sheet l FiledJune 11, 1964 N wt Oct 1967 M. BELCENGER 3, 5,6

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Oct. 3, 1967 v M. BELLENGER 3,345,634

RADIO INTERFEROMETER Filed June 11, 1964 3 Sheets-Sheet 5 Fig.5

50% ou L mioui/ycy TAM/VJLdTOR H FIX F 2 PHASE I COMP/1P4 T04 Claims.61. 343-113 The present invention relates to radio interferometers.

Radio interferometer systems generally comprise antennas spaced apart bya distance called base and equal to a few wavelengths of the incomingsignal. The antennas are respectively coupled to phase measurementdevices for determining the relative phase shift between the signalsreceived by each antenna. The knowledge of this phase shift enables todetermine the direction of a source, which is, for example, a spacevehicle.

Generally, the source concerned is extremely remote and thesignal-to-noise ratio is small, thus setting a limit to the systemsensitivity.

To increase sensitivity, phase correlation is generally used whichnecessitates the use of a third antenna. The latter is inter-posedhalfway between the two antennas, to give a third signal whose phase istaken as a reference.

However, under certain conditions, it is not possible to use a thirdantenna, for example for technological reasons.

It is an object of this invention to provide a two antenna radiointerferometer system. According to the invention, local means areprovided to produce a third signal, whose phase is controlled to be atany instant equal to the average of the phases of the signalsrespectively picked up by the two antennas.

The invention will be better understood from the following specificationand appended drawings wherein:

FIG. 1 is a block diagram showing the operating principle of aconventional interferometer;

FIG. 2 is one embodiment of the invention;

FIGS. 3 and 4 are explanatory graphs; and

FIG. 5 is a further embodiment of the invention.

In FIG. 1, two antennas 1 and 2 are separated by a base whose lengthwill be designated by 21. Half way between antennas 1 and 2, Le. at adistance I from each antenna, is placed a third antenna 3.

Antennas 1, 2 and 3 receive from a distance source a signal which is,for example, a pure sinusoidal wave. The

direction of the incoming signal makes an angle 0 with the base.

If the phase of a signal, as picked up by the antenna 3, is taken as aZero phase, the phases of the signal received by antennas 1 and 2 willbe respectively 2 I 2 and-I- with 41r cos 0 T x A being the operatingwavelength.

nited States Patent Ofifice 3,345,634 Patented Oct. 3, 1967 tector 12detects a lowirequency signal carrying the phase information (,0.

Such an arrangement performs a linear detection with a lowsignal-to-noise ratio threshold, i.e. for

S being the signal and B the noise.

As a matter of fact, the signal from antenna 3 can be consldered as acarrier having two side'bands The above procedure makes the use of athird antenna 3 necessary. However, under certain conditions, it mayhappen, that it is not physically possible to have a third antennamounted between the two other antennas, for example, due to a lack ofavailable room.

It is an object of this invention to achieve the same result withoutusing a third antenna.

The invention consists in controlling an oscillator in such a manner asto have constantly the same phase shift gu/2 with respect to signals sand s which are shifted with respect to each other by go, such that:

A first embodiment of the invention is shown in FIG. 2.

By way of example, it will be assumed that s is a sinusoidal wave with afrequency equal to 136 mc./s.

The system comprises two identical channels connected respectively toantennas 1 and 2. Antenna 1 feeds an amplifier HF; and antenna 2 anamplifier HF Mixers M and M are respectively coupled to amplifiers HFand HF They are also coupled to a single side band suppressed carriedfrequency generator 0L designated hereinafter as a frequency translator.

This system receives a signal s at a frequency of 50 c./s. and providessignals Mixers M and M provide thus at their respective outputs signalsof respective frequencies:

F 1O mc./s. 50 cJakg0 being the phase shift between signals s and s Bothsignals are fed to an amplifier F1 having as output signal f shown inFIG. 3 This signal is at a frequency of 10 mc./s. and is modulated at 50c./s. with phase (p/2 with respect to signal s This signal is passed toa mixer M Signals FIM11 and FIM12 are respectively fed to two furtheramplifiers F1 and F1 which pass them to mixers M and M afteramplification.

A second'local oscillator 0L delivers to mixers M M and M a signalhaving a predetermined phase at a frequency of 9500 kc./s=F

Mixers M and M put out intermediate frequencies signals havingrespective frequencies.

which signals are amplified by amplifiers F1 and F1 Amplifiers F and Fare respectively followed by mixers M and M fed by an oscillator 0L3,whose frequency is 500 kc./s. and whose phase is controlled by phasecomparator CP, through an amplifier A.

Output signal from mixers M and M are phaseshifted, by

7| 1T '4- andrespectively, by phase shifters D and D and arerespectively applied to the inputs of comparator CP. The system of FIG.2 operates as follows: Mixers M and M provide signals whose respectivefrequencies are:

1 131250 c./s.\/+g

As long as the phase shift provided by phase shifters D and D ismaintained, the phase of oscillator L3 is ::0, that is to say equal theaverage of the phases of signals s and s If such is not the case, acorrection voltage appears at the comparator output, and will bring backoscillator 0L3 to its correct phase. The reference signal of phase 0 hasthus been created.

Mixer M delivers a signal whose frequency is f =50O kc./s., which ismodulated at 50 c./s. /2 and is passed to amplifier F1 The latter iscoupled to a synchronous detector whose further input is fed by thereference signal from local oscillator 0L3.

Detector DC puts out a signal at 50 mc./ s.+ p/ 2 carrying the phaseinformation, which is passed to filter F, followed by a frequencydoubler DF, which delivers a signal at 100 c./s. with a phase +zp.

It is to be noticed that the described system is particularly suited tobe adapted to a receiver having two antennas and incorporating thechannels, ending at amplifier F123.

The system of the invention provides a linear detection of signalenvelope at 500 kc./s. and thus avoids the contamination of the signalby noise. Briefly, the operation comes down to elaborating a localsignal having the same frequency as signals s and s and whose phase isequal to the mean value of the respective phases of their signals.

FIG. 5 shows another embodiment of the invention. The same referencenumbers are used to designate the same components in FIGS. 5 and 2.

The difference between the system of FIGS. 2 and 5 consists in that inthe latter one of the local oscillators, i.e. oscillator 0L2, isincorporated in the phase control loop.

It results that signals amplified by the intermediate frequencyamplifiers, following said oscillator are brought back to the center ofthe band, the phase regulation compensating for the phase shift due tothe Doppler effect. Tfu's makes it possible to use amplifiers with anarrower pass band.

In the embodiment considered, oscillator 0L3 has a fixed frequency of500 kc./s. This frequency stabilization can be achieved by means of aquartz. Phase comparator CP controls the phase of the 9500 kc./s.oscillator 0L2, through an amplifier A.

It results that unit FI FI and so on furnishes a complex signalconfiguration as shown in FIG. 4, the phase control consisting incausing signals s and s to rotate together to obtain the desired phases.

Of course, the invention is not limited to the embodments shown anddescribed which were given solely by way of example.

What is claimed is:

1. A radio interferometer for determining the direction of a sourcetransmitting a signal comprising: two antennas for picking up saidsignal, with a first and a second phase respectively, local means forgenerating a local signal, and means for locking the phase of said localsignal to a phase value which is the average of said first and secondphases, further local means for generat ing, from said signals as pickedup by said antennas, a further signal having the frequency of said localsignal and a phase equal to the difference between said average and oneof said first and second phases; and phase comparing means, having twoinputs for receiving respectively said local and said further signal,and an output.

2. A radio interferometer comprising: a first and a second aerial forcollecting the signal from a remote source for measuring the phase shift(p between said signal as respectively collected by said first and saidsecond aerial; a first and a second chain, identical to each other,respectively connected to said first and second aerials, said first andsecond chains comprising respectively in series a plurality of mixersand amplifier stages and a first and second output, respectively;frequency translating means connected to said mixers of said first andsaid second chains for deriving from said signal, as respectively pickedup by said aerials, a first and a second wave of the same low frequencyhaving respective phase shifts with respect to a reference phase; phasecomparing means respectively connected to said first and second outputsand having an output delivering a control signal when said respectivephase shifts deviate from if f and 2 respectively; a third chain havingtwo inputs respectively connected to said first and said second chains,and comprising a plurality of series connected mixer and amplifierstages, for producing a third intermediate frequency modulated by saidlow frequency phase signal shifted by (p/ 2 with respect to saidreference phase; an oscillator phase locked to said reference phase bysaid phase comparing means; and a synchronous detector, connected tosaid third chain and to said oscillator, and having an output forproducing an output voltage proportional to (p/2.

3. A radio interferometer comprising: a first and a second aerial forcollecting the signal from a remote source for measuring the phase shift(,0 between said signal as respectively connected by said first and saidsecond aerial; a first and a second chain, respectively connected tosaid first and said second antenna, and a third chain, each of saidfirst and said second chains comprising in series a first mixer, a firstintermediate frequency amplifier, a second mixer, a second intermediatefrequency amplifier, and a third mixer; a frequency translator connectedto said first mixers for generating and feeding to said firstamplifiers, a first and a second wave, whose respective frequencies arethe sum and the difference of the frequencies of an intermediatefrequency wave and of a low frequency wave, respectively phase shiftedwith respect to a reference phase by 2 and 2 said third chain comprisingin series a mixer amplifier, a fourth mixer, and a third amplifier; saidmixer amplifier receiving said first and said second waves anddelivering a third intermediate frequency wave modulated at said lowfrequency with a 2 phase shift, with respect to said reference phase; alocal oscillator connected to said second mixers of said first and saidsecond chains and to said fourth mixer for feeding to said secondamplifiers of said first and second chains and said third amplifierrespectively fourth, fifth and sixth waves, respectively frequencytransposed from said first, second and third waves; a second localoscillator, having an output connected to said third mixers of saidfirst and said second chains; a phase comparator and two phase shiftersrespectively connected to said third mixers of said first and saidsecond chain and having respective phase shifts; a synchronous detectorhaving two inputs respectively connected to the output of said secondlocal oscillator and to said third amplifier and having an output; saidphase comparator having an output for controlling one of said first andsaid second local oscillator.

4. A system according to claim 3 wherein said phase comparator controlssaid first local oscillator.

5. A system according to claim 3, wherein said phase comparator controlssaid second local oscillator.

References Cited UNITED STATES PATENTS OTHER REFERENCES Mengel: Trackingthe Earth Satellite and Data Transmission by Radio, In Proceedings ofthe IRE, June 1956,

10 TK7800I7, pp. 755-760 relied on.

RODNEY D. BENNETT, Primary Examiner.

CHESTER L. JUSTUS, Examiner.

15 D. C. KAUFMAN, Assistant Examiner.

3. A RADIO INTERFEROMETER COMPRISING: A FIRST AND A SECOND AERIAL FORCOLLECTING THE SIGNAL FROM A REMOTE SOURCE FOR MEASURING THE PHASE SHIFT$ BETWEEN SAID SIGNAL AS RESPECTIVELY CONNECTED BY SAID FIRST AND SAIDSECOND AERIAL; A FIRST AND A SECOND CHAIN, RESPECTIVELY CONNECTED TOSAID FIRST AND SAID SECOND ANTENNA, AND A THIRD CHAIN, EACH OF SAIDFIRST AND SAID SECOND CHAINS COMPRISING IN SERIES A FIRST MIXER, A FIRSTINTERMEDIATE FREQUENCY AMPLIFIER, A SECOND MIXER, A SECOND INTERMEDIATEFREQUENCY AMPLIFIER, AND A THIRD MIXER; A FREQUENCY TRANSLATOR CONNECTEDTO SAID FIRST MIXERS FOR GENERATING AND FEEDING TO SAID FIRSTAMPLIFIERS, A FIRST AND A SECOND WAVE, WHOSE RESPECTIVE FREQUENCIES ARETHE SUM AND THE DIFFERENCE OF THE FREQUENCIES OF AN INTERMEDIATEFREQUENCY WAVE AND OF A LOW FREQUENCY WAVE, RESPECTIVELY PHASE SHIFTEDWITH RESPECT TO A REFERENCE PHASE BY